Effects of Nitrogen Topdressing Timing and Rate on Grain Yield and Protein Content in ‘Hokkai 266’, a Successor Line for the Winter Wheat Cultivar ‘Yumechikara’

Abstract

This study aimed to clarify the effects of nitrogen topdressing timing and application rate on grain yield and grain protein content in ‘Hokkai 266’, a successor line to the winter wheat cultivar for bread ‘Yumechikara’. Field trials were conducted in Memuro in 2021 (sown in 2020) and in Bihoro in 2022 (sown in 2021). Nitrogen topdressing treatments were designed by varying the allocation of fertilizer applied at the regrowth, spike formation, and flag leaf stages. ‘Hokkai 266’ produced more than 100 additional spikes m^–² compared with ‘Yumechikara’ at both sites and tended to show higher grain yield. Grain protein content increased significantly in treatments where topdressing at the regrowth stage was omitted or reduced and fertilizer allocation at the flag leaf stage was increased, compared with the control. Although no significant correlation was detected between grain yield and grain protein content, ‘Hokkai 266’ exhibited a strong negative correlation between spike number and grain protein content, suggesting that the increase in spike number is associated with reduced protein content. Furthermore, grain weight tended to decline in upper spikelets and in the second and third florets depending on fertilizer allocation. These findings suggest that the yield and protein content in ‘Hokkai 266’ may be increased by reducing nitrogen application at the regrowth stage and emphasizing topdressing at the flag leaf stage while avoiding excessive spike production.